Reflective polarizers are widely used in liquid crystal displays (LCD) backlights to increase image brightness and light utilization efficiency. The reflective polarizers usually serve as pre-polarizers for recycling the polarization component that would be absorbed by the absorbing polarizers of the LCD. Current LCDs utilize primarily two types of reflective polarizers, DBEF and DRPF, both based on birefringent polymers and sold by 3M. Polarizers having optical thin films on prismatic surface interfaces with a 90 degree apex angle have been also been proposed as reflective polarizers for LCD backlights. However, to date, such prismatic reflective polarizers have not been commercialized because of poor performance and high manufacturing cost.
FIG. 1, adapted from FIG. 3. of U.S. Pat. No. 5,422,756, illustrates a prismatic reflective polarizer 10 consisting of a light-entrance medium 12 having a microstructured surface 14 with a series of prismatic structures 16. A thin film optical coating 18, with alternating thin film layers of high refractive index material and low refractive index material, is situated on the microstructured surface 14. The optical coating 18 is designed to preferentially transmit one polarization of light and reflect the orthogonal polarization. An inverted structured surface 20 is optically coupled to the optical coating 18 and provides a smooth light-exit surface 22. Geffcken et al. first proposed this type of prismatic reflective polarizer 50 years ago in U.S. Pat. No. 2,748,659, issued on Jun. 5, 1956, “LIGHT SOURCE, SEARCHLIGHT OR THE LIKE FOR POLARIZED LIGHT.” Improvements to this approach, directed primarily towards applications in LCD backlights, have been disclosed by Ogura in U.S. Pat. No. 5,061,050, issued on Oct. 29, 1991, “POLARIZER,” by Weber in U.S. Pat. No. 5,422,756, issued on Jun. 6, 1995, “BACKLIGHTING SYSTEM USING A RETROREFLECTING POLARIZER,” and by Weber in U.S. Pat. No. 5,559,634, issued on Sep. 24, 1996, “RETROREFLECTING POLARIZER.
As shown in FIG. 1, light rays incident on the first sidewall 17a of the prismatic structures 16 are either transmitted by the optical coating 18, for p-polarized light 30, or reflected, for s-polarized light 32. P-polarized light 30 then exits the prismatic reflective polarizer 10 unperturbed. However, the once-reflected s-polarized rays 34 are reflected a second time by the second sidewall 17b of the prismatic structure 16. S-polarized light 32 is therefore retro-reflected back towards its origin as doubly-reflected s-polarized rays 36. For the prismatic reflective polarizer 10 to function, the apex angle 24 of the prismatic structure 16 should be between 80 and 100 degrees, preferably 90 degrees. Therefore, relative to light-entrance surface 21 of the light-entrance medium 12, the inclination angles 26 should be approximately 45 degrees.
The optical coating 18 contains a stack of alternating high-index and low-index layers that are quarter-wave thick, relative to the wavelength of light in the material. As disclosed by Weber in the '634 patent, for application to visible light in LCD backlights where light recycling is desired over the entire visible spectrum, the optical coating 18 has a number of sub-stacks. Each of the sub-stacks is designed with quarter-wave layers for a different portion of the visible spectrum. The design disclosed in the '634 patent has a total of 28 layers to cover the entire visible spectrum. Such a high layer count would be very challenging to manufacture at low cost for a large area. Furthermore, as described in '634, the angular performance of the coating limits the useful function of the prismatic reflective polarizer 10 to an angular range of approximately +/−10 degrees in air. The limited angular performance is due, in large part, to the fact that light is incident on the thin film optical coating 18 at angle of approximately 45 degrees with respect to the surface normal. As is well known in the art, polarizing optical film stacks are more difficult to design, and have a narrower performance range, for 45 degree incidence than at somewhat larger angles.
The prismatic reflective polarizers that have been previously disclosed have two significant challenges: 1) a severe angular and wavelength sensitivity, limiting the number of suitable applications, and 2) a large number alternating high and low refractive layers, leading to high manufacturing cost. Increasing the layer count can diminish wavelength sensitivity, but this remedy increases manufacturing cost even further. There is a need therefore for a prismatic reflective polarizer that works over a large range of incident angles and wavelengths and that can be manufactured at reasonable cost.